Image blur due to camera shake or fast moving objects is a common problem associated with recording images of scenes with a still image camera. It is generally desirable to shorten the exposure time by increasing the shutter speed of the camera to avoid image blur. Shortening the exposure time, however, lessens the amount of incident light that reaches the imaging plane of the camera. It is therefore necessary to open the aperture setting of the camera to maintain the same exposure value at the image plane. Opening the aperture setting, however, reduces depth of field thereby making it difficult to distinctly capture images over a broad distance range. Adjustments in shutter speed and aperture setting are therefore interactive, which makes it difficult to obtain quality images under certain scene conditions, as optimum exposure values must sometimes be sacrificed to provide sufficient depth of field or optimum depth of field must be sacrificed to provide a better exposure value. The difficulty of reaching an acceptable compromise between shutter speed and aperture setting is particularly compounded when long focal length "zoom" lens are employed, as the depth of field at a given aperture opening and subject distance is decreased, for long focal lengths, as compared to shorter focal lengths.
In film cameras, the tradeoff between shutter speed and aperture setting can be addressed by using a film having a different ISO rating. Electronic imaging cameras, however, generally employ an image sensor having a fixed "speed" or exposure response. It is therefore an object of the invention to provide an automatic exposure system for an electronic still camera that automatically provides the best possible still image for any given scene illumination level and lens focal length setting.